Illuviation

Illuviation, Accumulation of dissolved or suspended soil materials in one area or layer as a result of leaching (percolation) from another. Usually clay, iron, or humus wash out and form a line with a different consistency and color. These lines are important for studying the composition and ages of rock strata.

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soil particles the diameters of which are less than 0.005 millimetre; also a rock that is composed essentially of clay particles. Rock in this sense includes soils, ceramic clays, clay shales, mudstones, glacial clays (including great volumes of detrital and transported clays), and deep-sea clays...